Rotational dynamics of CO solvated in small He clusters: A quantum Monte Carlo study

Abstract

The rotational dynamics of CO single molecules solvated in small He clusters (CO@HeN ) has been studied using Reptation Quantum Monte Carlo for cluster sizes up to N = 30. Our results are in good agreement with the roto-vibrational features of the infrared spectrum recently determined for this system, and provide a deep insight into the relation between the structure of the cluster and its dynamics. Simulations for large N also provide a prediction of the effective moment of inertia of CO in the He nano-droplet regime, which has not been measured so far.

Rotational Study of Carbon Monoxide Solvated with Helium Atoms

“…With the addition of the next He atoms, the shift turns negative and leads toward its ultimate limiting value for He nanodroplets. The different behavior of the He N -CO clusters can be related to the propensity of the He atoms to cluster at the oxygen end of carbon monoxide [24]. The observed sign and linearity of the shift of the He N -CO cluster band origins is in qualitative agreement for this cluster size regime with recent theoretical simulation using the SAPT and CBS þ corr potentials, but quantitatively much closer to the values from the CBS þ corr potential.…”

“…It is quite interesting to compare the vibrational shift to the inferred value from the nanodroplet measurements [26]. If the constant increment observed for the vibrational shift for N ¼ 1-8 continues to larger N, then the inferred nanodroplet shift of À0:254 cm À1 would already be reached at N ¼ 10, before filling of the first solvation shell at N ¼ 14 [24]. Therefore, to attain its nanodroplet value, the vibrational shift must have a blue upturn at some larger N numbers or the band shift increment becomes smaller above N ¼ 8.…”

“…They were denoted as aand b-types in analogy with He-CO, in which the a-type transition corresponds to end-over-end rotation of the whole complex, and the b-type corresponds to nearly free rotation of CO within the complex [22,23]. Theoretical simulations of the IR results [24,25] showed that the existence of two series at small N is likely due to a larger asymmetry of the cluster in this size regime and that the b-type series eventually disappears because of increasing cylindrical symmetry in larger clusters.…”

Rotational spectroscopic study of hydrogen cyanide embedded in small 4He clusters

“…It appears that this behaviour is also reflected in the characteristics of the smaller He N -molecule clusters, where helium density decouples from the rotational motion of a light rotor at much smaller critical N-values than in the case of heavier molecules. For example, the turnaround point was found to be at N = 3 for the case of He N -CO. [11][12][13][14][15] The critical N-value and the evolution of B with N also depend sensitively on the He-molecule interaction potential, in particular its angular anisotropy. For example, the critical N values for He N -CO 2 , 16 He N -N 2 O, [17][18][19][20][21] and He N -HCCCN 22 are at N = 5, 8, and 9, respectively.…”

Spectroscopy and dynamics in helium nanodroplets

“…This is attributed as due to a significantly higher anisotropy of the interaction potential of CO with He compared to that of HCN with He. Quantum Monte Carlo calculations on clusters CO-He N (N=1-30) by Cazzato et al [146] rotational excitation energies combined with vibrational shift calculations by Paesani and Gianturco [245] are in excellent agreement with IR experiments on such cluster sizes [246]. These calculations found that the rotational excitation energy was almost constant after completion of the first helium solvation shell (N=14), and by extrapolation predicted a nanodroplet B value 78% of the gas phase value, considerably larger than what was found [244].…”